The invention relates to tubular threaded connections comprising a male threaded element located at the end of a first tubular component and connected by screwing onto a female threaded element located at the end of a second tubular component.
Such connections are used in particular to constitute casing strings or production tubing strings or drill strings for hydrocarbon wells or for geothermal wells.
The male and female threaded elements can each be located at one end of great length pipes, the male threaded element of a first pipe being screwed into the female threaded element of a further pipe to constitute an integral threaded connection.
Alternatively, the male threaded elements can be disposed at each of the two ends of great length pipes, these being connected by a short length pipe or coupling the ends of which carry two female threaded elements: such a connection between two great length pipes is known as a xe2x80x9cthreaded and coupled connectionxe2x80x9d and employs two tubular threaded connections.
More particularly, the invention relates to threaded connections known as premium connections, which comprise radially interfering metalxe2x80x94metal sealing surfaces associated with abutting surfaces intended in particular to precisely position said sealing surfaces.
Such premium threaded connections are described, for example, in European patent EP 488 912, and enable to ensure the seal of the threaded connections against fluids in different service configurations (axial tension or compression, internal or external pressure, bending, etc.).
Recent techniques for drilling deviated wells at an inclined or even horizontal angle necessitate rotating the pipes and threaded connections connecting them as they drop into the well.
Such techniques require making up the threaded connections at high makeup torques, always substantially higher than the torque employed when dropping into the well, failing which the position of the sealing surfaces might be altered and the connection might leak.
Because of the desired torques, and the bending loads in portions that are bent by the geometry of the well (deviated wells), the abutting surfaces are subjected to large loads.
In the more particular case of casing strings or tubing strings, a well is provided with a plurality of concentric strings of pipes and the internal and external diameters of the constituents of the strings are necessarily limited to admit a maximum number of strings into the well.
Abutting surfaces, which are surfaces on the threaded elements that are oriented substantially perpendicular to the axis, are thus of a limited radial thickness and in the case of such threaded connections, are subjected to very high stresses that may result in unacceptable plastication.
International patent documents WO 94/29627 and WO 00/14441 disclose tubular threaded connections allowing makeup at a very high torque, which cause the flanks of the threads to completely or partially act as abutments.
The aim of the present invention is to produce a premium tubular threaded connection with a pair of male and female abutting surfaces that are particularly resistant to plastication without causing the thread flanks to act as abutments.
The aim of the present invention is to be applicable to premium tubular threaded connections with one or more abutments for each threaded element, but in which the principal abutment (which comes into abutting contact first and is under the most stress) is produced on the male threaded element at its free front end, and in a corresponding manner on the female threaded element.
The aim of the present invention is also to be applicable to any type of thread, thread form or sealing surface form.
The tubular threaded connection of the invention comprises a male threaded element at the end of a first tubular component and a female threaded element at the end of a second tubular component.
The male threaded element comprises a male threading and is terminated by a male lip.
The male lip comprises:
an external peripheral surface on which a male sealing surface is formed;
a male annular abutting surface that is substantially transversely oriented close to and connected to the male sealing surface and constituted by the front surface of the free end of the male threaded element;
an internal peripheral surface.
The term xe2x80x9csubstantially transversely oriented surfacexe2x80x9d means both planar and non planar surfaces, for example conical surfaces, the generatrix of which does not extend more than 30xc2x0 with respect to a plane normal to the axis of the threaded connection.
The female threaded element comprises so as to co-operate with corresponding means on the male threaded element a female threading, an internal peripheral surface carrying a female sealing surface, and a female shoulder.
The female shoulder has a female annular abutting surface that is substantially transversely oriented, close to and connected to said female sealing surface and defining a female annular shoulder zone that is subjected to axial compressive loads as said male abutting surface bears against said female abutting surface.
The male threading is made up into the female threading until said male abutting surface bears against said female abutting surface, said male sealing surface then interfering radially with said female bearing surface.
In accordance with one feature of the invention, the female annular shoulder zone comprises an internal peripheral surface, the internal diameter of which is smaller than the diameter of the internal peripheral surface of the male lip at least in close proximity to said female abutment surface, the ratio R of these two diameters being less than 1 but higher or equal to 0.9.
Such a feature enables to reduce the equivalent von Mises stress in the female annular shoulder zone which is higher on numerous prior art threaded connections than the stress in the male abutment due according to the inventors to the tri-axiality of the stresses created by the neighbouring bearing surfaces: the principal stresses in the male abutment are all compressive while in the female annular shoulder zone, only the axial stress is a compressive stress, the two other principal stresses being tensile stresses.
This feature also enables to keep a sufficient internal diameter for passage inside the threaded tubular connection.
In some of their figures Patents FR 1 488 719 and FR 1 489 013 disclose a threaded tubular connection with abutting surfaces and sealing surfaces and provided with a male lip at the end of the male threaded element and a female shoulder on the female threaded element, the internal diameter of the internal peripheral surface of the female annular shoulder zone being in close proximity to said female abutting surface smaller than the diameter of the internal peripheral surface of the male lip.
No limit of value has been given in those two documents as regards the ratio of those diameters.
Furthermore in patent FR 1 489 013, no function is linked to the difference in internal diameter; it can only be assumed that the smaller diameter of the internal peripheral surface of the female annular shoulder zone results from the upsetting of the pipe for making the threaded female element.
In the case of Patent FR 1 488 719 dealing with threaded connections for casing tubes, the female annular shoulder zone has an internal diameter smaller than the male lip in order to avoid damaging the male lips and consequently the seal of the threaded connection by shocks during the dropping of the drill pipes inside the casing tube string. Thus the function of the difference in internal diameter is quite else than in the case of the present invention.
Preferably, ratio R satisfies the following relationship:
{square root over (1.7xe2x88x920.7S2)}xe2x89xa6Rxe2x89xa6{square root over (1.2xe2x88x920.2S2)}, 
in which S equals the ratio between the diameter of the external edge of said male abutting surface and the internal diameter of the male lip.
This feature tends to minimise the differences in equivalent stress between the male abutment and the female annular shoulder zone and thus to optimise the geometry of the threaded connection of the invention.
Advantageously, the internal peripheral surface of the female shoulder of the threaded connection of the invention has a minimum diameter at a point where said internal peripheral surface intercepts a tapered surface of maximum shearing defined as follows:
it is a tapered surface with a vertex half-angle of 45xc2x0 coaxial with the female threaded element;
its diameter decreases in the female shoulder zone as the distance increases from said female abutting surface; and
it passes through the external edge of said female abutting surface.
Advantageously again, at its end on the side of said female abutting surface, the diameter of the internal peripheral surface of the female shoulder is equal to the internal diameter of the internal peripheral surface of the male lip.
Preferably, the internal peripheral surface of the female shoulder comprises a first portion called a transition portion the internal diameter of which gradually decreases from said female abutting surface, and a second cylindrical portion with a minimum internal diameter.
A junction point can be defined between the transition portion and the cylindrical portion of the internal peripheral surface of the female shoulder.
Preferably again, this junction point is located substantially mid-way axially between on the one hand the end of said transition portion on the side of said female abutment surface and on the other hand the point of intersection of the internal peripheral surface of the female shoulder with the tapered surface of maximum shear.
Further advantages and characteristics of the invention will become clear from the following detailed description and accompanying drawings.